Library of Congress
Boston, Mass. Boston molasses explosion

Safety Lessons from the Great Molasses Flood of 1919

June 7, 2024
Discover the catastrophic events of the Great Molasses Flood, where engineering failures led to a deadly wave of syrup in Boston’s North End.

Various theories have evolved to explain exactly what went wrong.

The headline of The Boston Post on Jan. 16, 1919, read: “HUGE MOLASSES TANK EXPLODES IN NORTH END; 11 DEAD, 50 HURT.”

The previous day, a 50-foot-tall storage tank containing fermenting molasses ruptured, releasing 2.3 million gallons of syrup. According to a report from the U.S. Census Bureau, a 40-foot-high wave of molasses weighing 26 million pounds (13,000 tons) and moving at 56 km/hr flattened pretty much everything in its path and flushed victims into Boston Harbor. 

Built in 1915 by US Industrial Alcohol (USIA), the tank fermented molasses to produce industrial alcohol—a precursor to acetone used as a gelatinizing agent in cordite. Cordite was in huge demand as a ballistic propellant during WWI, and lucrative contracts were up for grabs.

So, what went wrong?

First, the tank was constructed using the wrong alloy and gauge of steel. Leakage tests, routinely carried out by filling such vessels to capacity with water, were never performed before the tank was commissioned. Supervisors and inspecting officials responsible for the tank’s construction also did not have the required engineering expertise. 

According to locals, the tank leaked from the start and reddish-brown stains from escaping molasses appeared on its blue paint job. Some reported collecting molasses from leaking rivets and seams for their own use. Others handed in pieces of metal to USIA that were thought to have sloughed off the tank.

The company’s response was to paint the tank brown to disguise leaking joints. 
For the four years it was in operation, residents in Boston’s North End neighborhood grew used to the almost constant groaning coming from the tank’s joints.  

The tank finally succumbed to physics on Jan.15 after a 600,000-gallon delivery of molasses pumped from a ship in Boston Harbor took it to capacity. 

The familiar groaning became a roar as the steel walls tore apart, and a tsunami of syrup engulfed the local area.

The final death toll reached 21, and 150 injuries were reported. The body of the last victim wasn’t recovered for another four months, and clean-up crews spent an estimated 87,000 hours removing the sticky syrup from streets, trains and remaining buildings. They also had to tackle horses, pedestrians and gawkers who had trodden it all over the city. For decades, the air in that part of Boston was sweetened with the odor of molasses on warm days.

The front page of The Boston Post on the same day has a short news item outlining the findings of an investigation carried out by the state police chemist in charge of explosives, W.L. Wedger. 

He is said to have reached the positive conclusion — by late evening of Jan. 15 — that the disaster was caused by an internal explosion, not a tank failure. 

Wedger’s investigation found the tank contained heating pipes that helped the molasses run freely into tank carts that trucked it to the company’s Cambridge distillery. 

After “a careful study into the matter,” Wedger reportedly concluded this heating arrangement “could generate a mixture of air and gas that would be as explosive as the same amount of air and gasoline.”

“That horses were blown about like chips, houses torn asunder and the heavy section of the elevated railway structure smashed like an eggshell were other considerations linked with the conclusion of Mr. Wedger,” noted the newspaper. 

Notwithstanding Wedger’s assessment, USIA countered a class action lawsuit against it by claiming the rupture was the result of a terrorist attack by anarchists. 

However, after six years of litigation, the Massachusetts Superior Court concluded the tank’s construction had been deficient. USIA was ordered to pay the victims’ families roughly $7,000 each. The $1-million total would be worth just over $18 million today. 

Over the next 105 years, various theories have evolved to explain exactly what went wrong. These include a fermentation process causing a build-up of carbon dioxide inside the tank that burst rivets from the holes that lacked the necessary reinforcements, and thermal shock caused by convective mixing between warm and cold molasses in the tank just prior to its expiration.

About the Author

Seán Ottewell | Editor-at-Large

Seán Crevan Ottewell is Chemical Processing's Editor-at-Large. Seán earned his bachelor's of science degree in biochemistry at the University of Warwick and his master's in radiation biochemistry at the University of London. He served as Science Officer with the UK Department of Environment’s Chernobyl Monitoring Unit’s Food Science Radiation Unit, London. His editorial background includes assistant editor, news editor and then editor of The Chemical Engineer, the Institution of Chemical Engineers’ twice monthly technical journal. Prior to joining Chemical Processing in 2012 he was editor of European Chemical Engineer, European Process Engineer, International Power Engineer, and European Laboratory Scientist, with Setform Limited, London.

He is based in East Mayo, Republic of Ireland, where he and his wife Suzi (a maths, biology and chemistry teacher) host guests from all over the world at their holiday cottage in East Mayo

Sponsored Recommendations

Keys to Improving Safety in Chemical Processes (PDF)

Many facilities handle dangerous processes and products on a daily basis. Keeping everything under control demands well-trained people working with the best equipment.

Comprehensive Compressed Air Assessments: The 5-Step Process

A comprehensive compressed air audit will identify energy savings in an air system. This paper defines the 5 steps necessary for an effective air audit.

Get Hands-On Training in Emerson's Interactive Plant Environment

Enhance the training experience and increase retention by training hands-on in Emerson's Interactive Plant Environment. Build skills here so you have them where and when it matters...

Managing and Reducing Methane Emission in Upstream Oil & Gas

Measurement Instrumentation for reducing emissions, improving efficiency and ensuring safety.